In the art of commercial airplanes, it is highly desirable to design airplane and engine configurations that yield reduced fuel burn per seat-mile, which is a metric of airplane fuel efficiency and carbon dioxide emissions. Carbon trading and Carbon tax regulations comparable to those already enacted in the European Union are also likely to be adopted in other industrialized nations including the United States. These environmental considerations become even more important in economic scenarios in which fuel cost increases. This motivates step-change technologies to reduce fuel consumption per passenger mile.
This need for reduced fuel burn per seat-mile may be in conjunction with anticipated near-term increases in stringency of community noise certification regulations. Current European workplace noise exposure regulations that affect allowable aircraft cabin noise work together with local airport environmental policies to also pose significant challenges to advanced propulsion design. Thus, improvements in community and cabin noise relative to existing airplanes are also desirable.
The emissions-based requirements motivate extremely high bypass ratio engines which can most easily be accomplished with un-shrouded engines. Some un-shrouded engines however might not have an optimized configuration for noise reduction. It is also an objective for commercial airplanes including their propulsors to be perceived in a positive way by the flying public, similar to how “jet airplanes” with turbofan propulsors are perceived in a positive way.
One existing approach to providing improved fuel efficiency or reduced fuel burn is to utilize turbofan engines with higher bypass ratios. However, very high bypass ratio turbofans suffer from large weight and drag penalties associated with their very large fan ducts. Very high bypass ratio turbofans also suffer from difficulties associated with achieving under-wing installations in low wing airplanes and difficulties in achieving simple lightweight thrust reversers.
Another existing approach to providing improved fuel efficiency or reduced fuel burn is to utilize a turboprop, propfan, or other “open rotor” types of propulsor. An open rotor propulsor is effectively a propeller with a six to ten discrete individual blades exposed at their tips, with a gas turbine core engine driving the propeller through a gearbox. Open rotor propulsors provide substantially better fuel burn through a higher effective bypass ratio and elimination of fan duct drag and weight, but may have airplane integration challenges, non-optimal community noise levels, and non-optimal cabin noise and vibration.
Thus, there is a need for improvements in turbine engine technology to provide high fuel efficiency, low emissions, low noise, and overall improved integration.